1. Field of the Invention
The present invention relates to a method of manufacturing inversion type ICs for high-density mounting and to an IC module in which inversion ICs are mounted.
2. Description of the Related Art
The construction of a conventional resin seal type IC 8 is shown in FIG. 18. A semiconductor chip 1 is mounted on a die pad. An electrode pad 2 on a surface 1a of the semiconductor chip 1 is electrically connected to an inner lead portion of a corresponding lead 4 by a wire 3. The semiconductor chip 1, the wire 3, the inner lead portion of the lead 4 and the die pad 11 are sealed by a main body of a package 5 made of a resin so that an outer lead portion 6 of the lead 4 is exposed. The outer lead portion 6 of each lead 4 is usually bent toward the bottom surface 1b of the semiconductor chip 1 in order to mount the IC.
When a plurality of ICs constructed as described above are mounted on a single mounting substrate and, for example, when a large-capacity storage device is formed, leads of the same pin numbers of a plurality of leads having the same functions must be connected to each other. When, for example, a plurality of ICs are mounted on both sides of a mounting substrate, since the pin arrangement of ICs mounted on the top surface of the mounting substrate does not match the IC pin arrangement mounted on the bottom surface, leads having the same functions do not match each other. Also, when a plurality of ICs are mounted on a single surface of a mounting substrate, leads having the same functions do not meet each other in adjacent ICs. As a consequence, there is a problem in that, since leads having the same functions are connected to each other, wiring on a mounting substrate becomes complex.
A method shown in FIG. 19 for solving such a problem is disclosed in Japanese Utility Model Laid-Open No. 62-168652. In this method, ICs, such as ICs 9, have inverted pin connections formed by bending the outer lead portion 6 toward the top surface 1a of the semiconductor chip 1, in contrast with standard ICs, such as ICs 8, in which the outer lead portion 6 is bent toward the bottom surface 1b of the semiconductor chip 1. A standard IC 8 is mounted on a top surface 7a of a mounting substrate 7, and an inversion IC 9 is mounted on a bottom surface 7b thereof. With this construction, leads corresponding to each other can be connected easily by merely providing a through hole 10 on the mounting substrate 7 because the leads of the same pin numbers of both ICs 8 and 9 are at the same point with the mounting substrate 7 in between.
However, if an ultra-thin package, such as a TSOP (Thin Small Outline Package) having a thickness of approximately 1 mm, is mounted as shown in FIG. 19, the following problem occurs. Since the package is thin, the outer lead portion 6 exposed to the outside from the main body 5 of the package is short. If heat stress, such as heat cycling, is applied thereto, the solder connection portion between the outer lead portion 6 and the mounting substrate 7 is destroyed by a short cycle. For this reason, although it may seem that, to absorb heat stress, for example, a lead guide surface of the IC 8 should be made higher and the length L1 from the lead guide surface to the mounting substrate 7 made longer, in the inversion type ICs 9, on the contrary, the length L2 from the lead guide surface to the mounting substrate 7 becomes shorter, and the performance deteriorates.
In addition, there is another problem in that, productivity is reduced since the shapes of the bent leads are different for ICs 8 and 9 and two types of dies for bending operations are required to manufacture standard and inversion type ICs.